Gap junctions are integral membrane proteins that enable the direct cytoplasmic exchange of ions and low-molecular-mass metabolites between adjacent cells. They provide a pathway for propagating and/or amplifying the signal transduction cascades triggered by cytokines, growth factors, and other cell signaling molecules involved in growth regulation and development. Dysfunctional intercellular communication via gap junctions has been implicated in causing many human diseases. The objective of this project is to use a multi-disciplinary approach to identify the key intrinsic regulatory mechanisms that are responsible for Cx43 and Cx45 function. The central hypothesis is that unique intermolecular interactions within the divergent CT domain of Cxs affect gap junction regulation. More specifically, we hypothesize that in the failing heart, Cx43CT phosphorylation alters protein partner interactions leading to remodeling of Cx43 from the intercalated disc, and that dimerization of Cx45 CTs is, in part, responsible for the channel properties of Cx45 that distinguish it from Cx43 and for the dominant-negative effect of Cx45 in heteromeric channels with Cx43. It is well-known that the CT domains of Cxs are key regulators of channel properties, and that dimerization of cytosolic domains are key regulators of ion channels. This proposal is significant because discovery of how interactions mediated by the CT domain can be modulated would open the door to strategies to ameliorate the pathological effects of altered Cx regulation in the failing heart. The following Specific Aims are proposed to investigate this concept: 1) Define how tyrosine kinases down regulate Cx43 gap junction intercellular communication, 2) Determine how Cx43 phosphorylation alters protein partner interactions, and 3) Identify the importance and mechanism of Cx45CT dimerization.